Is all tanzanite heat treated?

Although some dealers may tell you they have unheated stones, it is best to take this with a pinch of salt. The accepted wisdom is that all Tanzanite is heated. Unheated the stone is a brownish colour and only after heating does it go blue/violet. Some crystals have admittedly been heated in the ground and are already blue but will generally be heated again anyway as the color always improves further. Stones are heated at 600 degrees celcius in a gemological oven for a short time. Further information is available at <a href=http://www.lapigems.com/tanzanite.asp</a>